YNP elk should be rebounding, the wolf population has dropped significantly, in addition to eating each other and moving to other areas to find prey, those wolves are also being hunted and their numbers further reduced when they leave the park.
You will never see the YNP elk herd rebound to the levels of the early 1990s, unless the Elk Management Plan in MT is substantially revised.
If you read the plan, you will note that the YNP herd and adjacent Gardiner units are AT objective. FWP is mandated by law to manage AT or BELOW the objectives for the EMU. Any hopes for 20k elk again are about as likely as wishing for unicorns.